I-beams are well-known profiles designed for carrying static loads with a minimal own weight. An I-beam has a cross section similar to that of a capital “I” with top and bottom chords that are vertically spaced apart by a central web portion. A special type of wooden and/or wood like I-beam is used in architectural constructions. This type of I-beam is known as I-joist. I-joists are described, for example in U.S. Pat. No. 4,195,462 to Keller and U.S. Pat. No. 6,460,310 to Ford et al.
I-joists are configured for carrying maximum loads while keeping their own weight to a minimum. For that purpose, I-joists have top and bottom chords with enlarged cross sections where compressive and tensile stresses are at a maximum. A central web portion connects both chords and keeps them at a distance and in plane with the load direction. The central web is also symmetrically positioned with respect to both chords. Under load the chords tend to deflect in plane with the applied load and consequently in plane with the web. The web is configured to provide sufficient stiffness and strength against the deformation tendency of the chords.
The web has a relatively thin cross-section geometry, which results in a certain buckling tendency of it. The buckling tendency of the web portion is a major criterion for the over all load carrying capacity of an I-joist. The structural integrity of the web is often compromised in architectural constructions. To assure sufficient buckling resistance of the modified web, manufacturers may provide dimensional safety limits for maximum diameter and other critical dimensions for holes cut into the web. Unfortunately, such standards are often not met by the construction workers that are typically in charge of fabricating the holes into the web. In a progressing architectural construction where the I-joist is already hidden from view, it becomes difficult to control the holes cut into the webs. Therefore, there exists a need for assuring the I-joists static load capacities irrespective of the actually hole shape cut into a web. The present invention addresses this need.
Reinforcement brackets for modified structural beams are well-known in the art. For example, U.S. Pat. No. 5,519,977 to Callahan et al (1). describes a reinforcement bracket for modified sections of a wooden joists. The invention is configured for joists with rectangular cross section. Support is mainly provided by configuring the bracket as a profile protruding in direction of the beam and having bending resistance that is maximized in protrusion direction of the joist. An eventually increased buckling tendency of the modified joist is not addressed by the invention. More over, the bracket attached at the modified joist offsets the all over section modulus of the combined cross section of joist and bracket out of the load plane. As an unfavorable result, a modified joist section supported by Callahan's bracket may have a greater buckling tendency than the same modified joist section not supported by Callahan's bracket.
Another example for a reinforcement bracket is described in U.S. patent application Publication 2002/0121066 also to Callahan et al (2). There, the bracket of the above-described invention is modified to accommodate for material separations cut through the top chord of an I-joist. In general, the applicability of this device may be limited since material separations of the chords are highly questionable due to their tremendous negative effect on the joist's load carrying capacity. As is well-known in the art and dependent on the load carrying condition, building codes strictly mandate that transverse holes bored or cut into a joist must remain at a certain distance from the top or bottom of the joist. For an I-joist in particular, it is recommended by manufacturers to avoid cutting either of the cords of a load carrying I-joist section. In addition, the buckling increasing effect of the bracket becomes even more dominant where the remaining cross section of the modified I-joist is much thinner than the rectangular section of a conventional modified joist.
Also, in Callahan et al (2) the connection between the bracket and the I-joist relies substantially on screws or nails laterally attached to the remainder of the chords. Chords that are fabricated from vertically stacked, laminated wood are highly sensible to splicing initiated by horizontally attached nails or screws. Attaching a support device on the chords for the purpose of transmitting bending loads from the I-joist onto the bracket consequently may result in splicing of the chords. The splicing of the chords results in a further weakening of the modified I-joist section. The splicing also reduces the rigidity of connection between the bracket and the modified I-joist section. Therefore, there exists a need for a support structure that may be attached to a modified I-joist with reduced and/or without laterally attaching to the chord(s). The present invention addresses this need.